How to get the upper X% of an object and remove the rest
One way is with a boolean modifier.


Add a plane (&#8679; ShiftA> Mesh > Plane) and make it large enough to exceed the bounds of the target object.
Add a boolean modifier to the target object and specify the plane as the second object:



Also ensure the boolean operation is set to Difference or Intersect.
This plane can be moved around and the mesh will be cut and filled accordingly. By using the dimensions of the target object, it's not difficult numerically place the plane in order to remove e.g. 75% of the object:


Which side of the mesh is removed can be controlled by flipping the normal of the mesh (&#9096; Ctrl&#8679; ShiftN) in Edit mode (&#8633; Tab), or by switching between Difference and Intersect boolean operations.